Contents Summary 483 Introduction 483 Progress of research on pollen wall development 485 The developmental role of the special cell wall 487 Meiosis and the establishment of microspore symmetry 489 The origins of the exine during the tetrad stage 490 The free microspore stage to pollen maturation 495 Conclusions 495 Acknowledgements 496 References 496 Summary The outer pollen wall, or exine, is more structurally complex than any other plant cell wall, comprising several distinct layers, each with its own organizational pattern. Since elucidation of the basic events of pollen wall ontogeny using electron microscopy in the 1970s, knowledge of their developmental genetics has increased enormously. However, self‐assembly processes that are not under direct genetic control also play an important role in pollen wall patterning. This review integrates ultrastructural and developmental findings with recent models for self‐assembly in an attempt to understand the origins of the morphological complexity and diversity that underpin the science of palynology.

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Despite the importance of species discovery, the processes including collecting, recognizing, and describing new species are poorly understood. Data are presented for flowering plants, measuring quantitatively the lag between the date a specimen of a new species was collected for the first time and when it was subsequently described and published. The data from our sample of new species published between 1970 and 2010 show that only 16% were described within five years of being collected for the first time. The description of the remaining 84% involved much older specimens, with nearly one-quarter of new species descriptions involving specimens >50 y old. Extrapolation of these results suggest that, of the estimated 70,000 species still to be described, more than half already have been collected and are stored in herbaria. Effort, funding, and research focus should, therefore, be directed as much to examining extant herbarium material as collecting new material in the field.herbarium specimen | monograph | taxonomy A ccurate species recognition underpins our knowledge of global biodiversity (1-3). In recent years, the lack of taxono mic activity has led to increased political (4) and scientific calls (3) to invest in the science of taxonomy, which is fundamental for what we know about species-level diversity. The assumptions behind these demands are that increased resources would necessarily lead to increased taxonomic productivity and accuracy. Given finite resources, it is essential that scientifically sound criteria regarding where funds should most usefully be targeted are used to determine priorities for taxonomic research. It is therefore surprising that the processes of collecting, recognizing, and describing species are poorly understood and only rarely discussed (5-7) and that there is little research focused on the processes that result in the recognition of new species. Many groups of organisms are so poorly known that measuring any aspect of the discovery process suffers from lack of data. In terms of completing the species-level "inventory of life," the flowering plants are viewed as an attainable priority research target because they are already relatively well known and the final inventory is estimated to be only 10-20% from completion (8). Furthermore, plants are pivotal organisms for monitoring and measuring global biodiversity because they comprise a species-rich component of almost all habitats on earth (9). An enhanced scientific understanding of the discovery process for flowering plants could help define specific priorities for funding agencies and facilitate the meeting of global biodiversity targets. Here, we focus on the temporal dynamics of the lag between the collection of flowering plant specimens and their subsequent recognition and description as new species (7). For a representative dataset, the discovery time (I) between the date of the earliest specimen collected (C) and date the description was published (D) was calculated for each species (Fig. 1). ResultsDiscovery I ranged fro...

Reconstructing phylogeny is a crucial target of contemporary biology, now commonly approached through computerized analysis of genetic sequence data. In angiosperms, despite recent progress at the ordinal level, many relationships between families remain unclear. Here we take a case study from Lamiales, an angiosperm order in which interfamilial relationships have so far proved particularly problematic. We examine the effect of changing one factor-the quantity of sequence data analyzed-on phylogeny reconstruction in this group. We use simulation to estimate a priori the sequence data that would be needed to resolve an accurate, supported phylogeny of Lamiales. We investigate the effect of increasing the length of sequence data analyzed, the rate of substitution in the sequences used, and of combining gene partitions. This method could be a valuable technique for planning systematic investigations in other problematic groups. Our results suggest that increasing sequence length is a better way to improve support, resolution, and accuracy than employing sequences with a faster substitution rate. Indeed, the latter may in some cases have detrimental effects on phylogeny reconstruction. Further molecular sequencing-of at least 10,000 bp-should result in a fully resolved and supported phylogeny of Lamiales, but at present the problematic aspects of this tree model remain.

The common and cosmopolitan freshwater benthic diatom Sellaphora pupula (Kütz.) Mereschk. is a model system for studying the nature of species in microalgae; the biological significance of morphological variation in this species complex has been widely demonstrated. The aim of this study was to establish a two-gene phylogeny (18S rDNA and rbcL) for 23 Sellaphora taxa, including 19 S. pupula aggregate (agg.) demes or species, S. bacillum (Ehrenb.) D. G. Mann, and S. laevissima (Kütz.) D. G. Mann. A range of analyses on separate and combined data sets indicated that Sellaphora is a monophyletic group containing four major clades. Of the traditionally recognized species, S. bacillum and S. laevissima are natural groups, but S. pupula is paraphyletic or polyphyletic because S. bacillum groups with S. pupula ''small lanceolate'' and S. lanceolata. Thickened bars at the poles of valves within the core ''pupula-bacillum'' group may be a morphological synapomorphy; the fossil record suggests that this group is at least 12 million years old. Otherwise, there was no clear pattern in the distribution of different morphologies among the major clades; each clade was also heterogeneous with respect to mating system. More intensive and extensive sampling will doubtless uncover even greater diversity; the challenge lies in its interpretation. Our results demonstrate the limitations of paleoecological, ecological, and biogeographical research based on morphospecies.